Velocity factor

Typical velocity factors

Velocity factor is an important characteristic of communication media such as category 5 cables and radio transmission lines. Plenum data cable typically has a VF between 0.42 and 0.72 (42% to 72% of the speed of light in vacuum) and riser cable around 0.70 (approximately 210,000,000 m/s or 4.76 ns per metre).

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Some typical velocity factors for radio communications cables provided in handbooks and texts are given in the following table:

:{| class="wikitable" |- !style="width:5em;"| VF (%) !! Transmission line !! Center insulation |- | || open-wire line ("ladder line") || air insulated |- | || HJ8-50B 3 inch Heliax coaxial cable || air dielectric |- | || RG-8 Belden 7810A coaxial cable || gas-injected foamed high-density polyethylene |- | || RG-6 Belden 1189A coaxial cable, RG-11 Belden 1523A coaxial cable || |- | || RG-8X Belden 9258 coaxial cable || foamed polyethylene dielectric |- | || Belden 9085 twin-lead || |- | || RG-8/U generic || foamed polyethylene |- | || Belden 8723 twin shielded stranded twisted pair || polypropylene insulator |- | || RG-213 CXP213 || solid polyethylene dielectric |}

Calculating velocity factor

Electric wave

VF equals the reciprocal of the square root of the dielectric constant (relative permittivity), \kappa or \epsilon_\mathrm{r}, of the material through which the signal passes:

:\mathrm{VF} = { \frac{1}{\sqrt{\kappa}} } \

in the usual case where the relative permeability, \mu_\mathrm{r}, is 1. In the most general case:

:\mathrm{VF} = { \frac{1}{\sqrt{\mu_\mathrm{r}\epsilon_\mathrm{r}}} } \

which includes unusual magnetic conducting materials, such as ferrite.

The velocity factor for a lossless transmission line is given by:

:\mathrm{VF} = { \frac{1}{c_\mathrm{0}\sqrt{L'C'}} } \

where L' is the lineic inductance (inductance per length, in henries per metre), C' is the lineic capacitance (capacitance per unit length) between the two conductors (capacitance per length, in farads per metre), and c_\mathrm{0} is the speed of light in vacuum.

Optical wave

VF equals the reciprocal of the refractive index {n} of the medium, usually optical fiber.

:\mathrm{VF} = { \frac{1}{n} } \

References

References

1. Gottlieb, I.M., ''Practical RF power design techniques'', TAB Books, 1993, {{ISBN. 0-8306-4129-7, p.251 ('velocity factor')
2. ''[https://web.archive.org/web/20240712045911/http://www.generalcable.co.nz/getattachment/f81f14ee-7bec-4841-82d5-84b2df58b4f5/Velocity-of-Propagation.aspx Velocity of Propagation]'', General Cable Australia Pty Ltd, retrieved 2010-02-13
3. "velocity of propagation" in Walker, P.M.B., ''Chambers Science and Technology Dictionary'', Edinburgh, 1991, {{ISBN. 1-85296-150-3
4. IEEE 802.3 Clause 8.4.1.3 ''The minimum required velocity of propagation is 0.77 c.''
5. IEEE 802.3 clause 15.3.1.3 ''The propagation delay shall be ≤5 μs/km. (This is equivalent to a velocity of propagation of 0.67c.)''
6. IEEE 802.3 Clause 10.5.1.3 ''The minimum required velocity of propagation is 0.65 c.''
7. IEEE 802.3 Clause 14.4.2.4 ''The maximum propagation delay of twisted pair shall be 5.7 ns/m (minimum velocity of 0.585 × c).''
8. (2011). "The ARRL Handbook for Radio Communications". [[ARRL]].
9. Kaiser, Kenneth L.. (2005). "Transmission Lines, Matching, and Crosstalk". [[CRC Press]].
10. "HJ8-50B".
11. "Belden Global Catalog".
12. "8723 multi-conductor, shielded twisted pair cable".